Adhesives, inks and coatings are typically blended formulations consisting of high molecular weight polymers, resins, oils, waxes, pigments, solvents and other additives. In all of these formulations, it is well documented that the resins are major and crucial components needed to impart the formulation rheology and end-use performance characteristics. It is also well documented that resin molecular weight and molecular weight distribution, taken in combination with a resin's solubility characteristics, affect resin performance and the properties of products incorporating the resin. In general, in accordance with well accepted theory, resins having lower molecular weights have better compatibility and solubility properties relative to those having higher molecular weights for use in the above and other formulations. It is for these reasons that resins containing high molecular weight fractions do not work well in adhesive formulations. Also, the resins used for ink pigment flushing give better pigment wetting and therefore better flushing performance if they have a low average molecular weight and are free of a high molecular weight fraction. Thus, control of molecular weight and molecular weight distribution is important in the creation of high utility resin products.
It is to be noted that a reference to the molecular weight of a resin or polymer, in reality, is a reference to an average molecular weight because, with few exceptions, polymers are complex mixtures of molecules having different molecular weights. The mixture of different molecular weights is called polydispersity. The reason for polydispersity relates to the statistical variations inherent to polymerization processes and the purity of raw materials. The following mathematical expressions define the three different molecular weight averages that are routinely used to characterize resins and polymers:
M.sub.n =number average molecular weight=.SIGMA.N.sub.i W.sub.i /.SIGMA.N.sub.i, PA1 M.sub.w =weight average molecular weight=.SIGMA.N.sub.1 W.sub.i.sup.2 /.SIGMA.N.sub.i W.sub.i, PA1 M.sub.z =z average molecular weight=.SIGMA.N.sub.1 W.sub.i.sup.3 /.SIGMA.N.sub.i W.sub.i.sup.2,
where N.sub.i is the number or moles of a material having a molecular weight of W.sub.i.
The ratio of M.sub.w /M.sub.n, defined as the polydispersity index (PDI), is a measure of the heterogeneity of a polymer sample with respect to molecular weight. The greater the value of PDI, the greater the heterogeneity or "broadness" of molecular weight distribution. A truly monodispersed system has, by definition, a PDI of 1.0. The closer the PDI of a given polymer approaches the theoretical limiting value of 1.0, the narrower is its molecular weight distribution.
As can be seen from the above equations, the z average molecular weight emphasizes the highest molecular weight fraction of the polymeric sample. Accordingly, those resins having relatively low z average molecular weights are believed to exhibit better overall formulated performance characteristics.
Average molecular weight and molecular weight distribution data is typically determined by gel permeation chromatography (GPC). This technique, in combination with calculations made against the retention times determined for a series of primary molecular weight standards, affords a means of determining all of the aforementioned average molecular weights.
The thermal polymerization of dicyclopentadiene (DCPD) and modified DCPD monomer streams is commonly practiced. These resins find wide utility in the preparation of inks, adhesives and coatings, but their relatively low overall performance excludes their use in the high performance applications. A principal motivation for using DCPD resins is low cost and availability. Their limitation, for many applications, is that in spite of the use of modifiers, the resulting resins still contain a significant proportion of high molecular weight polymer. This high molecular weight fraction limits solubility and compatibility and ultimately the utility of the resins. These resins also typically have dark colors.
Limited solubility and compatibility severely restricts the use of the existing thermally polymerized DCPD-based resins in many potential applications. Substantial concentrations of olefinic, vinyl aromatic or other reactive modifiers, when used in combination with short polymerization times, limits the proportion of undesirable high molecular weight material in a resin. However the inherent disadvantage of this approach is that it gives low resin yields. U.S. Pat. No. 4,650,829 discloses such a short reaction time and low reaction temperature polymerization of dilute DCPD streams. While it does afford the desired low molecular weight resins, it would be expected to give low yields. Another major disadvantage to using DCPD, and in particular the commercially available DCPD streams, which can contain significant proportions of the vinyl aromatics modifiers, is that they give dark colored resins. Resins having Gardner colors of greater than about 7 are generally unacceptable for adhesive and coating applications. U.S. Pat. No. 5,171,793 also discloses conditions where short polymerization times, in combination with high concentrations of reactive modifiers, results in resins with desirable molecular weights, but unfortunately the yields are low and the final resins have very dark colors. The resins of this patent generally have Gardner 16 colors and require hydrogenation processes to produce the light colors required for adhesives.
The present invention provides a method for preparing resins having relatively low molecular weights and relatively narrow molecular weight distributions. The invention also provides a method for preparing resins having light colors acceptable for use in a wide range of adhesive applications. Furthermore, the invention provides resins which exhibit good performance in hot-melt and pressure sensitive adhesives. The invention further provides improved resins for pigment flushing applications and lithographic gel varnish printing ink applications.